With reference to FIG. 1, modern designs for integrated circuits (ICs) often call for one or more conductive, usually metallic, patterned, interconnection layers 10 separated from the semiconductive substrate 20 and each other by one or more insulating layers 30. These insulating layers, usually made of silicon dioxide or some other glassy material, are referred to as interlevel dielectric layers. In structures of this kind, vertical electrical connections are typically made between selected, vertically adjacent places on different conductive levels by opening a window 40, often referred to as a "via", in an interlevel dielectric layer before depositing the upper one of the two conductive layers that are to be joined. The via extends down to an underlying conductive layer or the semiconductive substrate. The sidewalls 50 of the via are coated with a conductor, typically during the deposition of the upper conductive layer. Ideally, this conductive coating should be thick enough to completely fill the via. The conductive material is typically an aluminum alloy, for example 98.5% aluminum, 1% copper, and 0.5% silicon by weight, deposited by a physical vapor deposition process such as vacuum evaporation or sputtering. It may be deposited directly on the insulator, or on top of a metallic barrier layer that is interposed between the insulator and the aluminum to prevent chemical reaction between the two.
There are economic pressures to increase the circuit density of ICs. For that reason, it is desirable to make the diameter at the top of the via as small as possible. However, considerations of current leakage and parasitic capacitance impose a lower limit on the thickness of the interlevel dielectric layer (thicknesses of 0.4-1.2 .mu.m are typical). As a result, the ratio of the insulating layer thickness to the via diameter, termed the "via aspect ratio", must be high, for example 1:1 or even 2:1.
Furthermore, it is desirable to make the via as nearly like a right circular cylinder as possible, rather than like a truncated cone. The cylindrical shape helps provide a sufficient area for contacting the conductive or semiconductive substance below. That is, because the contact conductance is generally proportional to the area of the bottom of the via, a via with a fixed top diameter will have greater contact resistance the more steeply its sidewalls are inclined.
One measure of the quality of the metallic coating of via sidewalls is the step coverage, defined as the ratio of the smallest metal thickness deposited on the sidewall, to the metal thickness on the upper surface of the interlevel dielectric.
Practitioners have encountered difficulties in filling high-aspect-ratio vias, or in coating them with high step coverage. Shadowing effects make make it difficult to deposit sufficient material on the sidewalls. The deposited alloy will readily fill the via only if two conditions are met: the metal must be sufficiently plastic for diffusion to be significant (generally requiring a temperature above about 300.degree. C.), and the metal must efficiently wet the surface of the insulating layer within and adjacent to the via.
In the absence of efficient wetting, the metal will flow into a discontinuous film of droplets having contact angles near 90.degree.. Surface tension can prevent these droplets from straddling the sharp corner at the mouth of the via, and can prevent them from moving efficiently down the sidewalls. On the other hand, in the absence of thermal diffusion, the metal will not flow significantly at all, and in particular will not flow efficiently down the sidewalls. Moreover, the alloy deposited on the upper surface of the insulating layer near the rim of the via will tend to form isolated beads 60 rather than coat the sidewalls. As these beads grow during deposition, they can shadow the sidewall even further and eliminate almost all direct deposition onto the sidewall. As a consequence, step coverage will fall below desired values. At worst, the beads can grow, bridge the via, and completely cut off any direct deposition to the sidewalls or bottom of the via.
A potential solution to this problem is to modify the composition of the deposited alloy to make it wet the insulator more efficiently. An appropriate additive is one that will reduce .sigma..sub.LV, which is the specific interface energy (SIE) between the alloy and the ambient atmosphere, relative to the difference .sigma..sub.SV -.sigma..sub.LS. In the preceding expression, .sigma..sub.SV is the SIE between the insulator (i.e., substrate) and the ambient atmosphere (i.e., vapor phase), and .sigma..sub.LS is the SIE between the alloy (i.e., liquid phase) and the insulator. Such an additive will reduce the contact angle, or even cause complete wetting if the alloy-ambient SIE is made less than .sigma..sub.SV -.sigma..sub.LS. The interfaces relevant to the definitions of .sigma..sub.LV, .sigma..sub.SV, and .sigma..sub.LS are shown schematically in FIG. 2.
Practitioners of the metallurgical arts have, in fact, considered problems of wetting at aluminum surfaces. For example, U.S. Pat. No. 4,254,189 issued to R. D. Fisher on Mar. 3, 1981, discusses metallurgical problems related to computer disc memories that include magnetically sensitive material, a substrate that may be aluminum or an aluminum alloy, and an intermediate layer between the substrate and the magnetic material. It is desirable for the intermediate layer to wet the aluminum substrate. An exemplary treatment for improving the wettability of the aluminum surface is to coat it with a monolayer of chromium or titanium, thereby increasing the interface energy of the aluminum surface. As a consequence, adhesion of glass or metal intermediate layers is improved.
As a further example, U.S. Pat. No. 4,450,207, issued to T. Donomoto, et al. on May 22, 1984, discusses the use of carbon or alumina fibers to reinforce an aluminum alloy. An alloy composition which includes 0.5%-4.5% magnesium is found to improve the wettability of the reinforcing fibers by the molten alloy. This is attributed to binding of magnesium atoms to the O and OH radicals attached to the surface of the reinforcing fibers.
The above examples show that practitioners have attempted, for some purposes, to improve aluminum adhesion by modifying the surface, or the bulk composition, of an aluminum body. However, practitioners have failed to provide a modification that leads to improved wetting of an insulating substrate by a deposited aluminum film.